1. Field of the Invention
The present invention relates to low level light sensing and amplifying devices and, more particularly, to low level light detectors having a quantitative protection device for protecting the detectors such as photomultiplier tubes from high intensity input while providing large dynamic range.
2. Description of the Prior Art
Under battlefield conditions, ambient light intensities can vary from brilliant sunlight to intensities that are lower than light provided by starlight, a range of more than seven orders of magnitude. Under some conditions low level light sensing devices, such as night vision devices, that operate in the visible or near infrared may be marginal in performance because ambient light illumination is very low. A device that can measure low levels of light intensity and that can function over a wide dynamic range and survive is needed to permit judgments about the trafficability of military vehicles using night vision devices. There is some evidence that some night vision devices lose sensitivity after prolonged periods of operation; therefore, it is important to know when the ambient light level is marginally low for tactical operations
The Geophysical Advisory Service of the West German armed forces has divided ambient light levels into six categories (shown in Table 1) that reflect different levels of troop mobility.
TABLE 1 ______________________________________ NIGHT BRIGHTNESS CATEGORY VERSUS ILLUMINATION LEVEL Category Light Level (mlx) Orientation Potential ______________________________________ 1 300 to 140 Satisfactory 2 139 to 40 Still adequate 3 39 to 5 Impeded 4 4.9 to 2 Substantially Impeded 5 1.9 to 1.0 Strongly Impeded 6 &lt;1.0 None ______________________________________
The lower bound which reflects any interest in troop mobility appears to be about 2.0 millilux (mlx). The equipment must also survive in daylight, which is about 10.sup.7 mix. In view of this very large range of light levels and the small number of categories, there is a need for a light measuring system of moderate accuracy and large dynamic range.
Ordinary silicon diodes do not provide reasonable signal-to-noise ratios under the lowest light levels without the use of detector cooling or mechanical choppers. Both approaches are deemed unsuitable for tactical applications. Photomultiplier tubes have adequate sensitivity down to 0.1 mix but are degraded by exposure to direct sunlight. There is also the requirement for covering a large dynamic range. Consequently, there is a need for a device that can sense and amplify light intensities over a wide range that do not degrade when exposed to direct sunlight.